A polyimide resin is a useful engineering plastic that has high thermal stability, high strength and high solvent resistance due to rigidity, resonance stabilization and firm chemical bond of the molecular chain thereof, and is being applied to a wide range of fields. A polyimide resin having crystallinity is further enhanced in the heat resistance, the strength and the chemical resistance thereof, and thus is expected for applications as alternatives of metals or the like. However, such a polyimide resin has high heat resistance, but on the other hand, disadvantageously exhibits no thermoplasticity and has low molding processability.
For example, Vespel (registered trademark), a highly heat-resistant resin, is known as a polyimide molding material (PTL 1). This resin is difficult to process by molding clue to its very low flowability even at a high temperature, and is also disadvantageous in terms of cost because it requires molding under conditions of a high temperature and a high pressure for a prolonged period of time. In contrast to this, a resin having a melting point and flowability at a high temperature, such as a crystalline resin, may be processed by molding easily and inexpensively.
Thus, a polyimide resin having thermoplasticity has been reported in recent years. Such a thermoplastic polyimide resin is excellent in molding processability in addition to the original heat resistance of the polyimide resin. The thermoplastic polyimide resin is therefore applicable to a molded article for use in an inhospitable environment to which nylon or polyester, a general purpose thermoplastic resin, is inapplicable.
For example, Aurum (registered trademark) is known as a thermoplastic polyimide resin (NPL 1). However, Aurum is limited by an available apparatus because this resin has a high melting point and requires a molding temperature of generally 400° C. or more.
A method using a long linear aliphatic diamine as a raw material diamine is one of the methods for improving the molding processability of the polyimide resin, i.e., the methods for decreasing the melting point of the polyimide resin (NPL 2). This reduces the rigidity of the polyimide resin, and thus also decreases the melting point. This method, however, might decrease the glass transition temperature along with the decrease of the melting point, and in particular, might reduce the strength at a high temperature. Another problem of this method is difficult synthesis of a polyimide resin using a raw material diamine composed mainly of an aliphatic diamine.
For the thermoplastic polyimide resin, it is desirable to impart various kinds of capabilities, for example, excellent mechanical strength, flame resistance, design properties, slidability, heat aging resistance, and conductivity, according to its use application.
As for imparting of mechanical strength, the thermoplastic polyimide resin may be reinforced with various kinds of fillers, such as glass fiber or carbon fiber. Among compound resins, a material containing a highly heat-resistant resin reinforced with a filler, such as glass fiber or carbon fiber, is increasingly being adopted in a wide range of fields that require high strength, high heat resistance, and long-term heat resistance, such as electronic members for surface mounting, members for automobile or aircraft engine peripherals, and members for ducts. With recent progress in technology, there has been a growing demand for strength or heat resistance. Since various characteristics, such as strength or heat resistance, are remarkably improved by the addition of various kinds of fillers, the compound resins have a high market share. However, the addition of a filler to Aurum is presumed to further reduce flowability and reduce molding processability.
As for imparting of flame resistance, Aurum is a wholly aromatic polyimide, and thus exhibits high flame resistance at a V-0 level in the UL94 standards. However, the problem of Aurum, as mentioned above, is a high melting point and low molding processability. On the other hand, if a long linear aliphatic diamine is used as a raw material diamine for improving the molding processability of the polyimide resin, the aliphatic site of the polyimide resin reduces resistance to thermal decomposition. In this case, flame resistance as very high as that of wholly aromatic polyimide resins is not exerted (NPL 3).
As for imparting of design properties, a polyimide resin is generally colored brown. This is due to the occurrence of intermolecular or intramolecular charge transfer. The coloring of the polyimide resin may become problematic in fields that require an excellent hue or in fields that require design properties, such as medical equipment, food manufacturing equipment, and substrates for solar cells.
Such coloring may be improved with various kinds of colorants. The polyimide resin assumes brown color mixed with redness and yellowishness. It is considered that the brown color may be canceled with a colorant of blue or green color, which is a complementary color of these colors. It is also considered that the polyimide resin-derived coloring may be suppressed by the addition of approximately several tens of mass % of a white colorant.
However, a general polyimide resin exhibits no melting point below the decomposition temperature. For obtaining a polyimide resin in which a colorant is uniformly dispersed, it is required to add a colorant at the stage of a polyamic acid solution, which is a precursor thereof. In this case, the final molded product is limited to a film or sheet shape. For a soluble polyimide resin, it is also necessary to add a colorant in the state of a polyimide resin solution. Hence, the final molded product is similarly limited by its shape.
A thermoplastic polyimide resin is useful because a colorant may be added during heat melting while a molded article may be produced in various shapes. However, Aurum is limited by an available colorant because it has a high melting point and requires a molding temperature of generally 400° C. or more, as mentioned above.
As for imparting of slidability, a polyimide resin generally exhibits favorable high-temperature sliding characteristics and is capable of intending higher high-temperature sliding characteristics by the addition of a slidability-improving agent.
A slidable material containing a highly heat-resistant resin supplemented with a slidability-improving agent is processed into gears, bearings, bearings, bushes and the like, and in particular, is increasingly being used in a wide range of fields that require high heat resistance, such as transport planes. Thus, its usefulness is high.
As for imparting of heat aging resistance, Aurum mentioned above is a wholly aromatic polyimide, and such a wholly aromatic polyimide generally exhibits high heat aging resistance. However, the problem of Aurum, as mentioned above, is a high melting point and low molding processability. If a long linear aliphatic diamine is used as a raw material diamine for improving the molding processability of the polyimide resin, the aliphatic site of the polyimide resin serves as a starting point for thermal degradation. Hence, this polyimide is far inferior in heat aging resistance to wholly aromatic polyimides (NPL 3).
As for imparting of conductivity, a polyimide resin is excellent in heat resistance, mechanical strength and the like. Studies have been made on use of the polyimide resin in fixing belts or intermediate transfer belts for use in, for example, copiers, printers, laser printers, facsimiles and their complex apparatuses, by mixing with a conductive material, such as carbon black. However, a general polyimide resin that exhibits no thermoplasticity requires complicated synthesis and film formation steps, and thus disadvantageously tends to aggregate by the addition of carbon black (see e.g., PTL 4). Aurum mentioned above is a thermoplastic polyimide resin, and its problem is a high melting point and low molding processability.
A composite material containing a thermoplastic resin combined with a fiber material is also known.
A composite material containing a resin combined with a fiber material, such as glass fiber, carbon fiber, or aramid fiber, has a light weight and high rigidity. A molded article using the composite material is therefore being used widely in mechanical parts, electric or electronic instrumental parts, parts or members for vehicles, instrumental parts for aircrafts or space, and the like. A thermosetting resin, such as an unsaturated polyester resin or an epoxy resin, or a thermoplastic resin, such as a polyolefin resin or a polyimide resin, is generally used as the resin for the composite material. The composite material using the thermosetting resin is advantageously excellent in physical properties, such as mechanical strength, but is disadvantageously unable to be remolded once molded by heating. For these reasons, a composite material using a thermoplastic resin that is remoldable by heating any number of times (see PTL 2 and PTL 3) has received attention, but is expected for further improvement in physical properties, such as mechanical strength.
A polyimide resin is known to be excellent in mechanical strength. The polyimide resin is a useful engineering plastic that has excellent mechanical strength as well as excellent thermal stability and solvent resistance due to rigidity, resonance stabilization and firm chemical bond of the molecular chain. The polyimide resin, however, generally has no melting point at a temperature lower than the decomposition temperature and exhibits no thermoplasticity. Accordingly, the polyimide resin, compared with the thermoplastic resins mentioned above, is difficult to prepare into a composite with a fiber material. Even if a composite material can be produced, the composite material disadvantageously has low molding processability.
Among polyimide resins, there exists a polyimide resin that exhibits thermoplasticity. For example, Aurum (registered trademark) is known as such a polyimide resin (see NPL 1). The material is a rigid wholly aromatic polyimide but succeeds to have a melting point, which is generally difficult to be observed, at a temperature lower than the decomposition temperature by introducing plural flexible ether bonds and meta structures into the structure. The material has a large number of flexible structures but has a high melting point (388° C.), which is peculiar to a polyimide, and in particular, a higher temperature exceeding 400° C. is required for molding the material. It can be said that there are difficulties in restrictions on equipment and handling property.
For improving the moldability, there have been some attempts of decreasing a melting point of a polyimide, but in practice, decreasing the melting point also decreases the glass transition temperature, thereby extinguishing the high glass transition temperature, which is peculiar to a polyimide. There is an empirical rule that the melting point and the glass transition temperature have a certain relationship (in general, there is often an approximate relationship, (glass transition temperature)/(melting point)=⅔ in terms of absolute temperature), and therefore decreasing the melting point for improving the moldability generally also decreases the glass transition temperature, which is one of the basic factors of the heat resistance.
For example, a method using a long linear aliphatic diamine as a raw material diamine is one of the methods for decreasing the melting point of a polyimide resin (NPL 2). This reduces the rigidity of the polyimide resin, and thus also decreases the melting point. However, the glass transition temperature is largely decreased along with the decrease of the melting point according to the aforementioned general rule, thereby extinguishing the high glass transition temperature, which is peculiar to the polyimide. Another problem of this method is difficult synthesis of a polyimide resin using a raw material diamine composed mainly of an aliphatic diamine.